Hydrogen is supplied to customers connected to a hydrogen pipeline system. Typically, the hydrogen is manufactured by steam methane reforming in which a hydrocarbon and steam are reacted at high temperature in order to produce a synthesis gas containing hydrogen and carbon monoxide. Hydrogen is separated from the synthesis gas to produce a hydrogen product stream that is introduced into the pipeline system for distribution to customers that are connected to the pipeline system. Alternatively, hydrogen produced from the partial oxidation of a hydrocarbon can be recovered from a hydrogen rich stream.
Typically, hydrogen is supplied to customers under agreements that require availability and on stream times for the steam methane reformer or hydrogen recovery plant. When a steam methane reformer is taken off-line for unplanned or extended maintenance, the result could be a violation of such agreements. Having a storage facility in connection with hydrogen pipeline operations to supply back-up hydrogen to the pipeline supply is therefore desirable. Considering that hydrogen production plants on average have production capacities that are roughly 50 million standard cubic feet per day or greater, a storage facility for hydrogen that would allow a plant to be taken off-line, to be effective, would need to have storage capacity in the order of 1 billion standard cubic feet or greater.
Additionally, there are instances in which customer demand can exceed hydrogen production capacity of existing plants. The storage facility allows excess hydrogen to be temporarily stored and subsequently available for back-up supply to assist in meeting customer demand when, for example, a steam methane reformer is unable to meet customer demand.
The large storage capacity can be met by means of salt caverns to store the hydrogen underground. Salt caverns are large underground voids that are formed by adding fresh water to the underground salt, thus creating brine, which is often referred to as solution mining. Caverns are common in the gulf states of the United States where demand for hydrogen is particularly high. Hydrogen storage has taken place where there are no purity requirements or less stringent (<95% purity) requirements placed upon the hydrogen product. In such case, the stored hydrogen from the salt cavern can be removed from the salt cavern without further processing.
However, utilizing a salt cavern to assist in the supply of higher purity hydrogen of at least 95% purity or greater is challenging. Stored hydrogen within the salt cavern has a tendency to become contaminated by intrusion of several contaminants, which can include, by way of example, water vapor, hydrocarbons (e.g., methane), sulfur-containing compounds and/or carbon dioxide. Contamination of the stored hydrogen requires removal of one or more contaminants from the stored hydrogen when withdrawn as a crude hydrogen stream from the salt cavern. The term “crude hydrogen” is intended to refer to one or more of the contaminants contained within a salt cavern which infiltrates through the walls of the salt cavern into the stored hydrogen to a degree where subsequent purification may be required prior to its return to a hydrogen pipeline to acquire a hydrogen purity level at or below a product purity specification. Methods have been implemented to ensure that impurities imparted by the salt cavern to the stored hydrogen do not deleteriously impact the hydrogen product in the pipeline. For example, U.S. Pat. No. 7,078,011 removes at least carbon dioxide and water vapor from a crude hydrogen stream withdrawn from a salt cavern to produce a hydrogen product stream having an impurity level at or below a product purity specification. U.S. Patent Pub. No. 2013/021349 removes crude hydrogen from a salt cavern and then dilutes the crude hydrogen with higher purity hydrogen from a hydrogen pipeline to form a resultant hydrogen product stream at or below a product purity specification. U.S. Pat. Nos. 8,425,149 and 8,757,926 and U.S. Patent Publication No. 2014/0241802 maintain a minimum quantity of stored hydrogen within the salt cavern to create a stagnant layer having carbon dioxide contained therein. The compressed hydrogen feed stream is introduced into the salt cavern and the stored hydrogen stream is withdrawn from the salt cavern without disturbing the stagnant layer to prevent carbon dioxide contamination transferring into the stored hydrogen stream, thereby allowing the stored hydrogen stream to be reintroduced into the hydrogen pipeline without carbon dioxide removal. Application Ser. No. 14/475,120 discloses a system and method for cooling a crude hydrogen stream that is withdrawn from the salt cavern to condense and separate the contaminants from the hydrogen. These methods disclosed in U.S. Patent Publication No. 2013/021349; U.S. Patent Publication No. 2014/0241802; U.S. Pat. Nos. 7,078,011; 8,425,149; 8,757,926; and Ser. No. 14/475,120, each of which is incorporated by reference herein in its entirety, require additional processing steps, which can add complexity to the hydrogen flow network that is in communication with the salt cavern, as well as potentially increase capital and operating expenditures.
Additionally, the ability to utilize a salt cavern to assist in the supply of higher purity hydrogen without leakage through the salt cavern walls is difficult based on the properties of hydrogen. Hydrogen is the smallest and lightest element within the periodic table of elements, having an atomic radius measuring 25 pm+/−5 pm. Consequently, higher purity hydrogen is typically considered one of the most difficult elements to contain within underground salt formations without measurable losses through the salt cavern walls. For example, storing large quantities (e.g., greater than 100 million standard cubic feet) of pure (e.g., 99.99%) gaseous hydrogen in underground salt caverns consisting of a minimum salt purity of 75% halite (NaCl) or greater without measurable losses of the stored hydrogen-from the salt cavern can present challenges. Methods for containing hydrogen within a salt cavern have been addressed. U.S. Pat. No. 8,690,476, which is incorporated by reference herein in its entirety, creates a permeation barrier along the walls of the cavern that allows high purity hydrogen to be stored therein. U.S. Patent Pub. No. 2014/0161533, which is incorporated by reference herein in its entirety, discloses monitoring and regulating the pressure of the stored hydrogen in the salt cavern between a predetermined lower limit and a predetermined upper limit.
As will be discussed, among other advantages of the present invention, an improved system and method for purging contaminants from a salt cavern is disclosed.